This invention concerns a method of freeze drying frozen and granulated coffee extract in a manner which results in a desirable dark product appearance. More particularly, it pertains to controlling the vacuum chamber pressure during the initial half of the freeze drying cycle to obtain a darker colored product.
Freeze drying concerns the removal of water from frozen products via sublimation in a vacuum chamber. In prior art processes for freeze drying coffee extract the chamber pressure has typically been maintained at less than 300 microns of mercury, (.mu.Hg.) and in many processes at less than 100.mu.Hg. throughout the drying cycle. The process of this invention deliberately maintains a higher chamber pressure in the initial portion of the drying cycle as a means of darkening the final product color.
The importance of color in a freeze dried product has long been recognized and an object of many research investigations has been to obtain a dark freeze dried soluble coffee which resembles regular roasted and ground coffee.
Prior art processes such as those described in U.S. Pat. Nos. 3,399,061 and 3,443,963 have recognized the importance of controlling the freezing profile of coffee extract as a means of obtaining a dark colored product. The disadvantage of such prior art processes is that one is restricted to the use of large, costly freezing equipment in order to obtain the necessary degree of control over the freezing profile.
In U.S. Pat. No. 3,244,529, Johnson et al. describe a process wherein after a dry shell has been formed in the freeze drying chamber around the particles being dried, a specified percentage of the remaining frozen water is allowed to melt and refreeze in the latter part of the drying cycle, as a means of darkening the product color. The disadvantage of such a process is that the melting of the inner ice surfaces is generally associated with flavor degradation. Also, the process is difficult to control and may result in a totally puffed and unusable product. Furthermore, when the process is maintained under control the amount of color darkening achieved is minimal.